My answer: Since Mark 15:48-50 breaks the Messianic Secret in proto-Mark, by having a Jesus who remembers to his enemies the fact that he was not an unknown person (i.e. unknown as a robber is by definition) but was someone *very well known* during the day. So it is an anti-marcionite interpolation.

So I have found another break of the Messianic Secret in proto-Mark by "Mark".

Following the logic of the previous comment above, I wonder if even the Barabbas episode breaks someway the Messianic Secret. Afterall, it is an attempt to identify with more precision Jesus by contrasting him against anohter man (Jesus Barabbas) who surely is not him (since he is a brigand and the true Jesus can't be a brigand).